1. Technical Field
The present invention relates to a heavy-load radial tire possessing improved bead durability.
2. Related Art
Pneumatic tires, when heavily loaded, have the ply main portions of their carcasses fall toward the rim. This involves substantial movement of the bead apex rubber, which in turn causes shearing strain between the bead apex rubber and the ply turnup portion. Repeated occurrence of shearing strain causes cord loosening at the outermost end of the ply main portion, resulting in bead damage. Bead damage is likely to occur especially on heavy-load radial tires, such as for trucks and buses, which involve heavy load.
In view of this, conventional heavy-load radial tires commonly use a two-layer structure as shown in FIG. 4A, where a bead apex rubber b disposed between a ply main portion a1 and a ply turnup portion a2 of the carcass is divided into a lower apex portion b1 of high-elastic rubber disposed on a radially inner side and an upper apex portion b2 of low-elastic rubber disposed on a radially outer side (i.e., see Japanese Patent Application Publication No. 2002-205508). With this structure, the high-elastic lower apex portion b1 reduces bead deformation and the low-elastic upper apex portion b2 alleviates stress to the carcass cord caused by shearing strain, thereby inhibiting cord loosening.
However, there is a need for further improvement in bead durability considering reuse as retreaded tires.
In view of this, Japanese Patent Application Publication No. 2008-037314 proposes, as shown in FIG. 4B, forming the high-elastic lower apex portion b1 of the bead apex rubber b into an L-shaped cross section composed of a bottom piece portion b1a and a standing piece portion b1b, the bottom piece portion b1a being disposed along a radially outer surface of a bead core c, the standing piece portion b1b standing from an axially inner end of the bottom piece portion b1a. This structure reduces the rubber volume of the lower apex portion b1 while increasing the rubber volume of the low-elastic upper apex portion b2. This provides for the advantage of enhancing alleviation effect for shearing strain especially acting on the outermost portion of the ply turnup portion a2. However, the L-shaped cross section of the lower apex portion b1 reduces rigidity, which increases the deformation quantity of the profile of the ply main portion while the vehicle is running.